Covered

No other bridge is more romantic or more a hallmark of the rural Vermont landscape
than the covered bridge. Beneath
picturesque gable roofs and rustic wood sheathing, intricately engineered truss
systems work to support the weight of the traffic passing through. A number of truss systems developed
during the era of timber frame bridge construction that greatly improved the
structural soundness. In order to
protect the timbers from the harsh Vermont winters, they were covered with the
distinctive walls and roof. The
majority of Vermont's covered bridges were constructed between 1820 and 1905,
and many were lost during the 1927 flood.
Following is a discussion of the common covered bridge types.

Multiple King-Post Truss

Many early covered bridge builders expanded the simple
king-post truss design of the open truss bridges. The multiple king-post truss was able to span moderate
crossings, occasionally exceeding 100 feet.

Burr Arch Truss

Theodore Burr, from Massachusetts, patented a new timber truss design
in 1806 and 1817 that would increase the span of covered bridges. Burr combined the multiple king-post
trusses with hewn timber arches.
The interior framing of a Burr arch truss is revealed in the image
below.

Burr Arch Truss: Image courtesy of Robert McCullough, Crossings: A History of Vermont
Bridges, 2005

Town Lattice Truss

Ithiel Town, an architect from Connecticut, patented one
of the most successful timber truss bridge designs in 1820. Town's design employs a cross-web, or
latticework, of diagonal timber planks.
The majority of Vermont's remaining covered bridges are classified as
town lattice trusses. The interior
latticework is exposed behind the wall opening in the image below.

Town Lattice Truss: Image courtesy of Robert McCullough

Long Truss

Stephen
H. Long received the first of many patents for bridge trusses in 1830. Long's truss system consisted of
rectangular panels each with two diagonals crossing to form an X. Long's truss, exposed in the image
below, never received as much acclaim as the trusses of Howe (discussed in the
next paragraph) or Town.

Long Truss: Image courtesy of Robert McCullough

Howe Truss

William
Howe, a railroad engineer from Massachusetts, patented the most widely used 19th
century bridge trusses in 1840 and 1846.
Howe's design was quite similar to Long's, except Howe replaced the
vertical timbers with iron beams.
An example of Howe's truss is exposed on the double-span bridge below.

Howe Truss: Image courtesy of Robert McCullough

Paddleford Truss

Peter Paddleford, a timber framer from Littleton, New
Hampshire, developed, but never patented, a timber truss design in the mid-19th
century. Paddleford's design was
also quite similar to Long's truss, except he combined short diagonals and
elongated diagonals into each panel.
A number of these bridges were constructed in Vermont.

Railroad Bridges

Railroad companies often used covered bridge designs
for their crossings. What is
notable, however, is that they increased the height of the traditional covered
bridge design and added monitor roofs, allowing enough room for smoke and steam
to escape. Eventually, around the
late 1880s, railroad companies recognized the shortcomings of timber frame
bridges, and they began constructing with iron.